Im not following you. The flex must play defense, if the flex is not on the field you would have an ureported substitution situation. The DP can play defense for anyone except the flex and it is not a substitution. If the DP does play defense for the flex then the flex has left the game and would have to use their reentry to come back.
Ok. I think I finally caught up. So you start the DP in the field with the Flex also on defense and another player who is hitting but not playing defense?